This invention concerns the conversion of the heating values of carbonaceous fuels into useful thermal, mechanical or electrical energy.
Burning coal to generate steam is one of the oldest of the industrial arts. Numerous inventions have been applied to improving its efficiency and alleviating the co-production of noxious smoke, which tends to contain unburned fuel, finely powdered ash and oxides of sulfur and nitrogen. Nevertheless, even with the latest technology, coal is considered a dirty fuel, capable only with great difficulty and expense of complying with increasingly stringent air pollution standards. Moreover, most conventional combustion apparatuses are quite inflexible with respect to variations in fuel properties, suffering loss of availability, capacity or emissions control if required to shift from one fuel source to another.
The high cost of removing sulfur oxides from conventional flue gases has resulted in a spread between the prices of low and high sulfur coals. Moreover, the former are found, for the most part, in western states remote from areas of greatest energy need. Thus, the market price structure provides incentive for the commercialization of a process able to produce steam and power from high sulfur coals without air pollution. Other reserves of solid fuels remain largely untapped because of high contents of water or ash.
Combustion of coal in conventional ways creates temperatures of 2.degree.-3000.degree. F. Conventional apparatus must, therefore, be constructed of expensive materials capable of withstanding such temperatures. Components of the ash frequently melt or sinter, forming deposits which foul parts of the apparatus, causing loss of efficiency, downtime and increased maintenance expense. A further undesirable consequence of the usual temperatures is the inadvertent formation of nitrogen oxides, pollutants which cannot be effectively and economically removed from flue gas with available technology.
Generation of high pressure steam does not inherently require such high temperatures since the boiling point of water at 2000 pounds per square inch is only about 635.degree. F. and at 3000 pounds per square inch under 700.degree. F.
It has been proposed to burn coal by the indirect means of first converting it to liquid or gaseous fuel, which can be desulfurized before combustion to a clean flue gas. These techniques also employ high temperatures and generally share serious economic and operational drawbacks associated with coal's tendency to cake and stick when heated, the formation of soot or tarry residues and difficulties with erosion and dust control. They are further burdened with low overall thermal efficiencies.
The catalytic effect of common alkalis such as soda ash (sodium carbonate) and limestone (calcium carbonate) on the reactivity of carbonaceous materials is well known and has been utilized in the gasification of coal and coke. Alkaline compounds are used in commercial steam-hydrocarbon reforming catalysts to promote the oxidation of carbon to gaseous products. Conventional combustions do not employ alkaline catalysts because, at the high temperatures, they would volatilize and/or combine with ash ingredients to form troublesome slag or clinker.
Some of the newer fluidized bed combustion processes do, however, use beds containing limestone, or similar alkaline particles, and thus are able to burn the fuel at reduced temperature, avoiding or minimizing nitrogen oxides and slag or clinker. Generally, two categories of fluidized be combusion processes are recognized in the art: atmospheric pressure and pressurized. While both are considered to have commercial promise, the atmospheric version requires a high excess of alkali to affect even moderately high sulfur removal and both encounter difficulty in separating dust from flue gas. Although pressurized fluidized beds achieve a better alkali utilization, sulfur removal is still incomplete and dust control is even more crucial since energy must be recovered from hot flue gases by expanding them through turbines subject to erosion.
It has been known for more than 70 years that water accelerates the reactions between coal and atmospheric oxygen. Ordinary combustion processes cannot take advantage of this phenomenon because wet fuel must be dried before it will ignite. Moreover, water entering a conventional combustion, as well as the known fluidized bed combustions, leaves the system as vapor, carrying with it as an energy loss its latent heat of evaporation.
The combustion-promoting effect of water is strikingly illustrated by a family of processes known as Wet Air Oxidation (WAO), which modify or destroy organic matter suspended in water by contact with air at elevated temperature and pressure. While used mainly to purify waste water WAO, which was originally known as the Zimmerman Process, has been proposed as a means of desulfurizing coal by partial oxidation and even for recovering energy from such fuels as peat. WAO is liquid phase and therefore confined to temperatures below the critical temperature of water (705.4.degree. F.), which limits reaction rates (requires large, expensive reactors) and the temperature at which useful heat can be delivered. WAO processes do not use alkaline catalysts.
In its capacity to modify potential fuels of hydrophilic nature, such as peat and biomass produced by aquaculture, WAO can comprise a useful pre-treatment in connection with my process. Much of the water, inseparable by ordinary methods, bound in such fuels is freed by this treatment, making it possible to charge them to my process in more concentrated form. Many hydrophilic fuels may be similarly modified by merely heating under pressure, after which excess water can be separated by mechanical or gravitational means.
Steam, flue gas, carbon dioxide, and mixtures thereof, are widely employed to enhance the recovery of residual oil from underground deposits. When steam is used alone the technique is known as steam flooding. Application of carbon dioxide is referred to as miscible flooding. Gas-steam mixtures stimulate production by pressure as well as heat. Conventionally, these recovery aids are generated by combustion of crude and refined petroleum oils. Even so, control of air and/or underground pollution is often a serious problem. There is a strong economic incentive for substituting solid fuels, which may cost only about one-third as much as petroleum, given an efficient combustion process which does not cause pollution.